# Copyright 2015 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================

"""A binary to train CIFAR-10 using multiple GPUs with synchronous updates.

Accuracy:
cifar10_multi_gpu_train.py achieves ~86% accuracy after 100K steps (256
epochs of data) as judged by cifar10_eval.py.

Speed: With batch_size 128.

System        | Step Time (sec/batch)  |     Accuracy
--------------------------------------------------------------------
1 Tesla K20m  | 0.35-0.60              | ~86% at 60K steps  (5 hours)
1 Tesla K40m  | 0.25-0.35              | ~86% at 100K steps (4 hours)
2 Tesla K20m  | 0.13-0.20              | ~84% at 30K steps  (2.5 hours)
3 Tesla K20m  | 0.13-0.18              | ~84% at 30K steps
4 Tesla K20m  | ~0.10                  | ~84% at 30K steps

Usage:
Please see the tutorial and website for how to download the CIFAR-10
data set, compile the program and train the model.

http://tensorflow.org/tutorials/deep_cnn/
"""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

from datetime import datetime
import os.path
import re
import time

import numpy as np
from six.moves import xrange  # pylint: disable=redefined-builtin
import tensorflow as tf
import cifar10

FLAGS = tf.app.flags.FLAGS

tf.app.flags.DEFINE_string('train_dir', '/tmp/cifar10_train',
                           """Directory where to write event logs """
                           """and checkpoint.""")
tf.app.flags.DEFINE_integer('max_steps', 1000000,
                            """Number of batches to run.""")
tf.app.flags.DEFINE_integer('num_gpus', 1,
                            """How many GPUs to use.""")
tf.app.flags.DEFINE_boolean('log_device_placement', False,
                            """Whether to log device placement.""")


def tower_loss(scope, images, labels):
    """Calculate the total loss on a single tower running the CIFAR model.

  Args:
    scope: unique prefix string identifying the CIFAR tower, e.g. 'tower_0'
    images: Images. 4D tensor of shape [batch_size, height, width, 3].
    labels: Labels. 1D tensor of shape [batch_size].

  Returns:
     Tensor of shape [] containing the total loss for a batch of data
  """

    # Build inference Graph.
    logits = cifar10.inference(images)

    # Build the portion of the Graph calculating the losses. Note that we will
    # assemble the total_loss using a custom function below.
    _ = cifar10.loss(logits, labels)

    # Assemble all of the losses for the current tower only.
    losses = tf.get_collection('losses', scope)

    # Calculate the total loss for the current tower.
    total_loss = tf.add_n(losses, name='total_loss')

    # Attach a scalar summary to all individual losses and the total loss; do the
    # same for the averaged version of the losses.
    for l in losses + [total_loss]:
        # Remove 'tower_[0-9]/' from the name in case this is a multi-GPU training
        # session. This helps the clarity of presentation on tensorboard.
        loss_name = re.sub('%s_[0-9]*/' % cifar10.TOWER_NAME, '', l.op.name)
        tf.summary.scalar(loss_name, l)

    return total_loss


def average_gradients(tower_grads):
    """Calculate the average gradient for each shared variable across all towers.

  Note that this function provides a synchronization point across all towers.

  Args:
    tower_grads: List of lists of (gradient, variable) tuples. The outer list
      is over individual gradients. The inner list is over the gradient
      calculation for each tower.
  Returns:
     List of pairs of (gradient, variable) where the gradient has been averaged
     across all towers.
  """
    average_grads = []
    for grad_and_vars in zip(*tower_grads):
        # Note that each grad_and_vars looks like the following:
        #   ((grad0_gpu0, var0_gpu0), ... , (grad0_gpuN, var0_gpuN))
        grads = []
        for g, _ in grad_and_vars:
            # Add 0 dimension to the gradients to represent the tower.
            expanded_g = tf.expand_dims(g, 0)

            # Append on a 'tower' dimension which we will average over below.
            grads.append(expanded_g)

        # Average over the 'tower' dimension.
        grad = tf.concat(axis=0, values=grads)
        grad = tf.reduce_mean(grad, 0)

        # Keep in mind that the Variables are redundant because they are shared
        # across towers. So .. we will just return the first tower's pointer to
        # the Variable.
        v = grad_and_vars[0][1]
        grad_and_var = (grad, v)
        average_grads.append(grad_and_var)
    return average_grads


def train():
    """Train CIFAR-10 for a number of steps."""
    with tf.Graph().as_default(), tf.device('/cpu:0'):
        # Create a variable to count the number of train() calls. This equals the
        # number of batches processed * FLAGS.num_gpus.
        global_step = tf.get_variable(
            'global_step', [],
            initializer=tf.constant_initializer(0), trainable=False)

        # Calculate the learning rate schedule.
        num_batches_per_epoch = (cifar10.NUM_EXAMPLES_PER_EPOCH_FOR_TRAIN /
                                 FLAGS.batch_size)
        decay_steps = int(num_batches_per_epoch * cifar10.NUM_EPOCHS_PER_DECAY)

        # Decay the learning rate exponentially based on the number of steps.
        lr = tf.train.exponential_decay(cifar10.INITIAL_LEARNING_RATE,
                                        global_step,
                                        decay_steps,
                                        cifar10.LEARNING_RATE_DECAY_FACTOR,
                                        staircase=True)

        # Create an optimizer that performs gradient descent.
        opt = tf.train.GradientDescentOptimizer(lr)

        # Get images and labels for CIFAR-10.
        images, labels = cifar10.distorted_inputs()
        batch_queue = tf.contrib.slim.prefetch_queue.prefetch_queue(
            [images, labels], capacity=2 * FLAGS.num_gpus)
        # Calculate the gradients for each model tower.
        tower_grads = []
        with tf.variable_scope(tf.get_variable_scope()):
            for i in xrange(FLAGS.num_gpus):
                with tf.device('/gpu:%d' % i):
                    with tf.name_scope('%s_%d' % (cifar10.TOWER_NAME, i)) as scope:
                        # Dequeues one batch for the GPU
                        image_batch, label_batch = batch_queue.dequeue()
                        # Calculate the loss for one tower of the CIFAR model. This function
                        # constructs the entire CIFAR model but shares the variables across
                        # all towers.
                        loss = tower_loss(scope, image_batch, label_batch)

                        # Reuse variables for the next tower.
                        tf.get_variable_scope().reuse_variables()

                        # Retain the summaries from the final tower.
                        summaries = tf.get_collection(tf.GraphKeys.SUMMARIES, scope)

                        # Calculate the gradients for the batch of data on this CIFAR tower.
                        grads = opt.compute_gradients(loss)

                        # Keep track of the gradients across all towers.
                        tower_grads.append(grads)

        # We must calculate the mean of each gradient. Note that this is the
        # synchronization point across all towers.
        grads = average_gradients(tower_grads)

        # Add a summary to track the learning rate.
        summaries.append(tf.summary.scalar('learning_rate', lr))

        # Add histograms for gradients.
        for grad, var in grads:
            if grad is not None:
                summaries.append(tf.summary.histogram(var.op.name + '/gradients', grad))

        # Apply the gradients to adjust the shared variables.
        apply_gradient_op = opt.apply_gradients(grads, global_step=global_step)

        # Add histograms for trainable variables.
        for var in tf.trainable_variables():
            summaries.append(tf.summary.histogram(var.op.name, var))

        # Track the moving averages of all trainable variables.
        variable_averages = tf.train.ExponentialMovingAverage(
            cifar10.MOVING_AVERAGE_DECAY, global_step)
        variables_averages_op = variable_averages.apply(tf.trainable_variables())

        # Group all updates to into a single train op.
        train_op = tf.group(apply_gradient_op, variables_averages_op)

        # Create a saver.
        saver = tf.train.Saver(tf.global_variables())

        # Build the summary operation from the last tower summaries.
        summary_op = tf.summary.merge(summaries)

        # Build an initialization operation to run below.
        init = tf.global_variables_initializer()

        # Start running operations on the Graph. allow_soft_placement must be set to
        # True to build towers on GPU, as some of the ops do not have GPU
        # implementations.
        sess = tf.Session(config=tf.ConfigProto(
            allow_soft_placement=True,
            log_device_placement=FLAGS.log_device_placement))
        sess.run(init)

        # Start the queue runners.
        tf.train.start_queue_runners(sess=sess)

        summary_writer = tf.summary.FileWriter(FLAGS.train_dir, sess.graph)

        for step in xrange(FLAGS.max_steps):
            start_time = time.time()
            _, loss_value = sess.run([train_op, loss])
            duration = time.time() - start_time

            assert not np.isnan(loss_value), 'Model diverged with loss = NaN'

            if step % 10 == 0:
                num_examples_per_step = FLAGS.batch_size * FLAGS.num_gpus
                examples_per_sec = num_examples_per_step / duration
                sec_per_batch = duration / FLAGS.num_gpus

                format_str = ('%s: step %d, loss = %.2f (%.1f examples/sec; %.3f '
                              'sec/batch)')
                print(format_str % (datetime.now(), step, loss_value,
                                    examples_per_sec, sec_per_batch))

            if step % 100 == 0:
                summary_str = sess.run(summary_op)
                summary_writer.add_summary(summary_str, step)

            # Save the model checkpoint periodically.
            if step % 1000 == 0 or (step + 1) == FLAGS.max_steps:
                checkpoint_path = os.path.join(FLAGS.train_dir, 'model.ckpt')
                saver.save(sess, checkpoint_path, global_step=step)


def main():
    cifar10.maybe_download_and_extract()
    if tf.gfile.Exists(FLAGS.train_dir):
        tf.gfile.DeleteRecursively(FLAGS.train_dir)
    tf.gfile.MakeDirs(FLAGS.train_dir)
    train()


if __name__ == '__main__':
    tf.app.run()
